A cellular mobile telephone has two modes of operation, an active mode and an idle mode. In the active mode, which corresponds to an exchange of dialog during the course of a communication established between the base station and the cellular telephone, the transmission and reception pathways of the telephone are both used to transfer voice or digital data between a base station and the telephone.
Outside of a telephone communication, the cellular telephone stands by in the idle mode. In the idle mode, the transmission and/or reception pathways are activated and deactivated at regular intervals to maintain a regular connection with the base station.
From a structural point of view, apart from the transmission/reception stage, which comprises the transmission pathway and the reception pathway, the cellular telephone comprises a processing stage connected to the transmission/reception stage and comprises means for digital modulation/demodulation. The means for digital modulation/demodulation may be partly provided using hard-wired logic and partly provided using software within a signal processor. This processor thus performs, in particular, the demodulation of the signal received and the modulation of the signal to be transmitted.
Moreover, various reference signals having chosen frequencies and clock signals must be generated and delivered respectively to the transmission pathway, to the reception pathway and to the modulation/demodulation processor. The frequencies of the reference signals fix or set the transmission and reception frequencies.
The various reference signals and clock signals are delivered by a frequency synthesizer stage controlled by automatic frequency control means. The accuracy required with respect of the transmission frequency is typically ±0.1 ppm for a nominal frequency in the 2 GHz range.
Currently, the internal architecture of the frequency synthesizer stage of a cellular mobile telephone is composed of conventional phase-locked loops which generate at the output of the oscillators the reference signals intended for the transmission/reception stage as well as the clock signal intended for the modulation/demodulation means.
The base signal intended for these phase-locked loops, which forms a frequency reference, is generated a controlled oscillator, such as a temperature-stable voltage-controlled oscillator (VTCXO oscillator), for example, having the required accuracy of about ±0.1 ppm. This oscillator conditions the accuracy demanded, and in particular, with respect to the transmission frequency.
On the basis of this architecture, the controlled oscillator is always operating, as is the phase-locked loop generating the clock signal for the modulation/demodulation means and the loop generating the reference signal for the transmission pathway. However, such a structure exhibits drawbacks. Specifically, not only is a controlled oscillator of this kind expensive, but its electrical consumption impairs the autonomy of the telephone.